External phosphorus load estimates and P-budget for the hydroelectric reservoir of Bort-Les-Orgues, France

This paper presents two methods used to estimate the external phosphorus lo ad for the hydroelectric reservoir of Bort-Les-Orgues (in France's 'Massif Central' region). The P load calculated from export coefficients (150 t yr( -1)) agreed with field measurements taken during a one-year survey carried out from March 1996 to February 1997 (121 t yr(-1)). Export coefficients re present a good option for estimating phosphorus loads of other reservoirs i n the region of Bort-Les-Orgues, provided that they display similar charact eristics. These external load estimates are greater than the maximum permis sible load for a mesotrophic lake calculated from the OECD equations (40 t yr(-1)); thus, a yearly reduction of about 60 t should ideally be achieved. This target, however, is probably too ambitious, and measures to reduce bo th point and non-point sources must be implemented for the reservoir to tbe mesotrophic. Moreover, Bort-Les-Orgues is a tourist area. As such, the pre sence of algal blooms (a regular occurrence on the lake) due to an excess o f P in the lake's water inflow could actually exert a negative impact on re gional economy.